Recent concerns about fouling problems caused by biofilms affecting optical oceanographic instruments have highlighted the need for a better understanding of their nature and extent in the marine environment. Glass slides were deployed in April and August for periods of up to 3 weeks at 5, 10, 20, 40, 80 and 160 m in the water column of Loch Fyne, Clyde Sea, W Scotland. Biofilms were enumerated using epifluorescence and bright field microscopy. During April the biofilm community varied significantly with depth, although this effect was attributable solely to changes in the diatom community. Diatom numbers peaked between 10 and 20 m. During August the biofilm community also showed a significant depth effect, although in this case there were significant effects for diatoms, rods, and filamentous bacteria. Cell numbers for diatoms, filamentous bacteria, and rod shaped bacteria peaked at 5 m. There was a significant linear relationship between the number of diatoms and bacteria on the slides deployed in August. No such relationship was found for the April data. The results indicate that optical performance may be significantly degraded after a few weeks and highlights the need for provision of suitable strategies to protect such surfaces from biofilm accumulation.